AC-30's have a poorly designed cathode follower. The bias point is totally wrong and crushes one side of the signal considerably. IMO either the cathode resistor should be 100K or the plate resistor of the preceding stage should be higher, 200K. The clipping is extremely asymmetric. Too much IMO. A little asymmetry is good because it warms things up. Too much causes excessive even-order harmonics which makes things fuzzy and indistinct. The clipping is so asymmetric on an AC-30 that it's almost a half-wave rectifier.
You can tweak this by adjusting the Preamp Bias point and/or lowering the Cathode Follower Compression. Or you can lower the Harmonics value which reduces the asymmetric distortion. The downside of that is that it then overdrives the phase inverter causing blocking distortion from excessive bias excursion. Another thing to try is to increase the Grid Clipping value which will add a little headroom. Start with the Bias point. AC-30's are very sensitive to the tube type and part tolerances. A tiny change in the bias point can make a big difference. The default bias point is based on Mullard ECC83 tubes.
I prefer the AC-20 because it doesn't have a cathode follower so doesn't suffer from these problems. The cathode follower in an AC-30 doesn't even do all that much. Normally you use a cathode follower to preset a low-impedance source to the tone stack but the tone stack in an AC-30 doesn't present that great of a load anyways.
Hi Cliff, you mentioned before that by adjusting the CF parameters in a non CF amp, we are essentially adding a CF to that amp. How would you characterize the effect of this on a high gain amp, like the Energy Ball or any other non CF high gain amp?